oil pressure

[wizwilson]

Have fitted an oil pressure gauge to the 0.3 bar location on my DJ engine, but at 2000 rpm have only got about 1.2 bar, as this is after the big ends am main bearings guess it will be lower than where they suggest checking at the 0.9 bar switch, no lights flashing but oil pressure drops to about 0.5 bar on tick over ?

any ideas ?

[kevtherev]

electric? or an accurate mechanical gauge?
what oil is being used?

with 20/50 mineral I get 18 psi or 1.25 bar at 900 rpm with a hot engine, on my mechanical gauge fitted at the 0.3 bar site (pushrods) with an 80 deg stat.
On a cold engine it's 60 psi or 4.1 bar at tickover.

At 65 mph I get 44 psi or 3 bar at 3400 rpm (hot)

it may be your gauge isn't accurate, a mechanical gauge will tell you for sure.

[itchyfeet]

Bentley tells you to measure pressure at the low switch.
should be 2 bar at 80 degc at 2000 rpm
It can get alot hotter than 80 and and pressure will drop when very hot.

my cheapo electric guage is not brilliantly accurate , it's ok mid range but low below 1 bar, as said a wet guage calibration is needed, also without temperature guage the readings are not so helpful.

[silverbullet]

I am of the opinion that 2 bar hot is still quite low for an engine with so few bearings to feed.
If it were any other engine I would want to see 2.5 bar or 40psi hot at idle, at least 3 bar at 2k rising to 4 bar by 3k.
Do all flat 4 have a weak relief spring just to try and limit oil temps because they never had an effective cooler?

[itchyfeet]

How does a weak spring help reduce oil temperature?
pressure is lower when hot.

[kevtherev]

You will get 40 psi at the mains hot, I do.
These figures are after that feed, and from the lifter gallery.

[wizwilson]

Hi itchyfeet, have the same gauge as you, and looking at your RPM would say the same sort of readings, it currently has Halfords 20/50 classic with a mann filter, both changed last week.

[itchyfeet]

Hi itchyfeet, have the same gauge as you, and looking at your RPM would say the same sort of readings, it currently has Halfords 20/50 classic with a mann filter, both changed last week.

Do you have a rev counter or are you estimating revs?
how hot was it? I think you need to take it for at least a 20 miniute drive around normal roads to be 80 degrees.
do you ever get the buzzer tripping?

You could try changing the oil pump, cover plate, pressure relief spring and piston, I got a little more pressure doing that but beware oil pump gaskets are too thick, need to make one out of thin paper.
Chances are engine is just high mileage as many are (including mine)

[wizwilson]

Using the vehicle rev counter, was after normal drive approx 10 miles, buzzer not tripping, have been looking on web they seem to do up rated pressure relief valve for T2 cant find one for T3, also looking at oil pump cover plate. think the engine has approx 60000 on it as it was replaced about 8 years ago.

[kevtherev]

Ok so what' the spec on this new engine ?
Have you a build sheet for it?

[silverbullet]

How does a weak spring help reduce oil temperature?
pressure is lower when hot.

Basic fluid dynamics. Higher relief pressure means that the pump can do more work on the oil; if the oil is considered as an incompressible fluid with constant volume then the temperature must rise with temperature. This has nothing to do with the breakdown of oil film strength and viscosity, which causes the catastrophic loss of pressure.
Two things are happening at the same time.

Our friend 10centlife has said this time and again: the oil pump is the biggest source of heat input to the engine oil.

VW were very conservative in their specifications to minimize the chance of engine failure and warranty claims, so a "just strong enough" pressure relief spring to limit the heat input to the oil and later on a marginal "interwarmer" to get some control when things got a bit too warm with the higher output of the 2.1 efi engine.

That's my take on it anyway...

[itchyfeet]

Ok interesting, but I like to work with empirical information

I had always thought the oil pressure relief only really was triggered in unusual situations such as blocked filter or perhaps on cold startup but it seems not, hence my question.

So looking at a piston and the crankcase it looks like there must be around 9mm of travel before the oil starts hemorrhaging out from slots down the side of the bore , before that is has just the slop in the bore to get past.

the piston is 16mm diameter, thats 0.31 sq inch

the spring when pressed 9mm give approx 3kg on my scales, thats 6.6lb

so it looks like at 20 or so PSI the valve starts to relief

any one see any errors in this calculation?

[silverbullet]

Numbers are always good Looks like a sound calculation to me.

Hang on... it doesnt take account of the spring preload when installed, which needs to be measured, of course.

[itchyfeet]

Numbers are always good Looks like a sound calculation to me.

Hang on... it doesnt take account of the spring preload when installed, which needs to be measured, of course.

Yes indeed, I'm on it...

[silverbullet]

I think this all comes back to the 10centlife pressure relief gallery mod i.e. preventing the valve discharge from going into the oil pump pickup and deliberately knobbling its performance.
Pretty sure that Vanistan engines make a constant 40psi minimum, rain or shine and with no fluctuations.